In radio reception, the linearity of the receiver is an essential requirement. The linearity performance of a receiver, in general, is usually dominated by the down-conversion mixer circuits. Such mixer circuits are used to translate or convert received high frequency signals down to a lower or intermediate frequency (IF). This conversion is achieved by mixing the received signals with a locally generated oscillator signal. By choosing the local oscillator signal to be a constant amount away from a selected or wanted signal in a first frequency band, the selected or wanted signal always appears at the same frequency in the intermediate frequency band. Thereby, selection of the selected or wanted signal may be facilitated by a fixed-tuned IF filter.
In homodyne or direct conversion receivers, the chosen intermediate frequency band is DC or zero frequency. The local oscillator then has a zero frequency separation from the selected or wanted signal. Any modulation on the selected or wanted signal that causes spectral components both above and below the nominal signal frequency becomes folded at the mixer output, as a component below the signal frequency or above the signal frequency will appear at the intermediate frequency above the nominal of zero. To allow for resolution of such folded components, two mixers are provided in a direct conversion receiver using local oscillator signals that are phase offset by 90 degrees. The components above and below the nominal signal frequency then appear folded as a sum signal at one mixer output and a difference signal at the other mixer output where they may be separated if desired. Such direct conversion receiver operations are described in more detail in document U.S. Pat. No. 5,241,702.
However, due to non-linearities, spurious responses will be generated in the direct conversion receiver, the worst being modulation-frequency interference at the receiver's mixer output caused by a strong amplitude-modulated signal of another transceiver. This will appear even if the frequency of the interfering signal considerably deviates from the receiving frequency. These interferences are mainly caused by the second-order distortion component which contains a variable-level DC component proportional to the amplitude of the interference-causing signal. The variable amplitude signal produces at the mixer output a signal which comprises a variable DC component and the frequency of which is identical with the variation of the amplitude. The spurious frequencies may corrupt the radio reception by blocking the following signal processing stages or deteriorating the detection of the desired signal which is overwhelmed by distortion.
The spurious frequencies can be categorized to exist due to the odd- and even-order non-linearities. The even-order mixing results are suppressed by using balanced or double-balanced mixer topologies. Ideally, the even-order spurious frequencies are cancelled in balanced and double-balanced constructions. However, in practice, the canceling is imperfect. The reason is the imperfect balance due to the mismatch of respective components in the differential branches, i.e. manufacturing tolerances.
In radio receivers utilizing a direct conversion architecture or a significantly low IF, the spurious signals cannot be removed by selecting an optimal IF. Due to the existence of the even-order distortions and imbalance in the circuitry, a variable DC component proportional to the signal level and amplitude modulation depth of the interfering signal occurs. Moreover, envelope distortions are detected, the amplitude of which is also proportional to the amplitude modulation depth of the interfering signal, and the frequency of which equals to the variation of the amplitude. Thus, not only a DC offset but also a low frequency disturbance may be generated to corrupt the desired reception band. This is a particular concern in down-conversion mixers of direct conversion receivers.
Several solutions for reducing distortions in radio receivers with low IF have been proposed. Document U.S. Pat. No. 5,749,051 suggests compensating unwanted terms caused by second-order intermodulation by feeding instantaneous power measurements to a signal processing unit along with the complex baseband signals. The signal processing unit then determines a complex compensation coefficient by correlating the power signal with the complex baseband signals. The complex compensation co-efficient is then employed to subtract a weighted amount of the power signal from the complex baseband signal in order to cancel the unwanted second-order intermodulation distortion terms. Furthermore, document EP 0 951 138 discloses a method for attenuating spurious signals in mixer circuits by setting variable-level bias voltages and/or currents to transistors in the mixer circuits. Additionally, document GB 2 346 777 suggests switching a DC offset correction in or out of the circuitry according to the received signal strength or signal-to-noise ratio.
Furthermore, the use of a dynamic matching procedure is described by E. Bautista et al., “Improved Mixer IIP2 through Dynamic Matching”, in the Digest of ISSCC 2000, pp. 376-377. According to this procedure, any undesirable second-order intermodulation distortion product generated in the mixer circuit is modulated to a frequency where it can easily be filtered off. This can be achieved by applying a periodic signal to input switches of the mixer circuit in order to modulate the received input signal. If the periodic signal is replaced by a pseudo-random signal, the undesirable second-order intermodulation distortion products can be spread over a wide range of frequencies to achieve a desired second-order input intercept point (IIP2) performance.
The second-order distortion phenomena itself and its undesired products, i.e. DC offset and envelope distortions, have not been thoroughly investigated so far. Due to lack of proper analysis of this topic, most of the solutions have been focused on the removal of the DC offset. However, even if the DC offset at the output of the mixer circuit is reduced to zero, the circuit may still be in an imbalanced condition, due to the fact that the envelope distortion itself causes a DC term which is related to other DC offsets in a complex manner.